Widely used prior art spark lightning arresters comprise a plurality of spark gaps (also called discharge gaps), each gap consisting of a pair of electrodes (a typical spark gap design is described, for example, in High Voltage Equipment, ed. by D. V. Razevig, 1976, Energy Publishers, Moscow, p. 297, FIG. 16-10). Examples of lightning arresters comprising several spark gaps are given on page 299 of said book and in Russian Patent No. 2,096,882 owned by the assignee of the present application. In case a large number of spark gaps are used, said gaps can be grouped into spark units, with some of said units comprising greater than one spark gaps (as shown in FIG. 16-13 of the above-cited book).
For lighting protection of high-voltage electric equipment, arrester assemblies consisting of series-connected arresters of a lower voltage class are also used (as described, for example, in High Voltage Equipment, ed. by D. V. Razevig, 1976, Energy Publishers, Moscow, p. 301, FIG. 16-14).
Arresters comprising a chain of spark units and arrester assemblies provide a long flashover path; therefore their use prevents lightning flashover from developing into a power arc, so that the electric installation protected by such arrester or such assembly continues uninterrupted operation. However, as the spark units are connected in series, a lightning overvoltage applied to the arrester is distributed among its spark units. As a result, the arrester's discharge voltage is on the whole much higher than that of one individual spark unit, and for that reason it is often difficult to ensure a desired low level of overvoltage limitation.
In a impulse mode, the voltage distribution among the spark units is determined by their own capacities and by their capacities relative to earth. In other words, the arrester comprising series-connected spark units constitutes a capacitive chain. Surge voltage is very unevenly distributed over such a chain, which results in a cascade break-down of all the spark units, with a sequential break-down of each of discharge gaps of the individual units.
An example of the cascade lightning arrester is an impulse spark lightning arrester comprising a first clamp and a second clamp for connecting the arrester to components of a power transmission line or an electric installation, which clamps are under a high and low potential, respectively; and a chain of N series-connected spark units, each comprising a discharge gap formed by a first electrode and a second electrode electrically connected to the input and output of said spark unit. The input of the first spark unit and the output of the Nth spark unit are connected to the first and the second clamp, respectively (see High Voltage Equipment, ed. by D. V. Razevig, 1976, Energy Publishers, Moscow, p. 303, FIG. 16-16).
A time front of lightning overvoltage impulse has duration of about 1 μs (microsecond), which is equivalent to the frequency f of alternating voltage of approximately 200 kHz. For such a high frequency, the resistance xC of an additional shunt capacitor C is quite small, since this resistance is inverse to the impulse frequency: xC=½πfC. As an example, for C=200 pF, the resistance of the shunt capacitor is about 4 kOhms. Therefore, due to the presence of the additional capacitor, the second electrode of the discharge gap of the first spark unit becomes connected to the ground via a relatively small resistance. Thus, voltage applied to the chain of spark units becomes applied practically entirely to the first gap alone. Meanwhile, other spark units in the chain are not subjected to any voltage. Under the impact of the applied voltage, the discharge gap of the first spark unit breaks down, and the entire voltage, due to the presence of the second shunt capacitor, becomes now applied to the second spark unit, and so on. Therefore, the arrester's triggering under application of a low voltage is ensured. However, the described cascade scheme, based on the sequential response of the spark units forming the chain, results in that the total response time of the arrester T becomes equal to the sum of response times t of all N single spark units: T=t1+t2+ . . . +tn, that is a substantial increase of said response time takes place.
As a consequence, the voltage—time characteristic of the prior art arrester in the domain of fast response times (about 1 μs) is quite steep, and this prevents the use of said arrester for protection of facilities with a flat voltage—time characteristic, such as cable links or transformers, when they are exposed to steep overvoltage impulses, since the arrester's response time is long enough for the impact overvoltage to grow to values dangerous to the insulation of the protected facility.
After the lightning impulse is over, the spark unit chain, due to the flashover of all spark units, remains exposed to an industrial voltage of 50 Hz frequency. At this stage, an even distribution of voltage over all the spark units is advantageous for more efficient extinction of an electric arc resulting from a follow-up current in each unit. However, at 50 Hz frequency, the resistances of the additional capacitances C are rather large, so they do not have a notable effect on the voltage distribution over the spark units.
Another example of a cascade spark arrester is the impulse spark arrester for overvoltage protection according to SU 1,669,026. An arrester disclosed therein comprises a first clamp and a second clamp for connecting the arrester to components of a power transmission line or of an electric installation, which clamps are under a high and low potential, respectively; and a chain of N (N=odd number equal to or greater than 3) series-connected spark units. Each unit comprises at least one discharge gap formed by a first main electrode and a second main electrode, which electrodes being electrically connected to an input and an output, respectively, of said spark unit, the input of the first spark unit and the output of the Nth spark unit being connected to the first and the second clamps, respectively. The prior art arrester further comprises N−1 resistors, the output of each odd spark unit, except the last one, being connected to the second clamp via one of said resistors, while the output of each even spark unit is connected to the first clamp via another of said resistors. This arrester, which has some features in common with the arrester of the present invention, essentially solves the problem connected to the cascade break-down. Nevertheless, there still remains a need for improvements in the design of the arresters of this type.